A steering angle sensor detects a steering angle of a steering wheel of a vehicle and provides a signal necessary for a vehicle control system to calculate a steering wheel angle, which is used in many vehicle control systems. When the vehicle is not in use, the ignition switch is powered off, and there is typically no power provided to the steering angle sensor. In rare instances the steering wheel may be turned when the vehicle is powered off, and the many vehicle control systems that rely on a known steering wheel angle will be affected. If a discrepancy exists between the steering wheel angle at the time the vehicle is powered off and the steering wheel angle at the time the vehicle is started up again, vehicle control systems may be adversely affected. Examples of vehicle systems that rely on accurate steering angle include electronic stability control, roll stability control, electric power steering, active park assist, advanced braking systems, steerable headlamps, and advanced accident avoidance systems, among others.
In order to combat this problem, a solution has been provided in which continuous power is supplied to the steering angle sensor, regardless of the state of the ignition switch. Typically the continuous power is supplied by the vehicle's battery, thereby consuming valuable battery power when the ignition is turned off. Another proposed solution is to store a learned angle at the time the vehicle is powered off and retrieve the stored angle at power on. The stored angle is a tentative angle until which time the vehicle moves and can confirm or reject the stored angle as a correct steering angle. Still another solution has been proposed in which power is intermittently supplied to the steering angle sensor and the sensor periodically detects the steering wheel angle. This solution conserves power but introduces the likelihood that a change in the steering angle may be missed should the change occur during a period of no power.
There is a need for a reliable solution to collecting a steering angle even during periods when the vehicle is powered down without wasting valuable battery power.